Indian Ocean Dipole

Why in news?

• A report by the Australian Bureau of Meteorology said that the IOD index has risen beyond the positive threshold.

About Indian Ocean Dipole (IOD):

• Indian Ocean Dipole (IOD) is an atmosphere-ocean coupled phenomenon in the Indian Ocean, characterised by a difference in sea-surface temperatures.
• IOD is the difference between the temperature of eastern (Bay of Bengal) and the western Indian Ocean (Arabian Sea).
• Indian monsoon depends upon not only El Nino La Nina but also IOD and other such ocean phenomena.

Characteristics of IOD:

• Temperature difference:
  • IOD occurs because of temperature difference. This temperature difference results into pressure difference which results in flowing of winds between eastern and western parts of Indian Ocean.
• Development:
  • IOD develops in the equatorial region of Indian Ocean from April to May, peaking in October.

Three Phases:

• The IOD has three phases such as Neutral, Positive and Negative IOD.
  • Neutral Phase of IOD:
    • During this phase water flows from the Pacific between Indonesia’s islands, keeping seas warm to the northwest of Australia. Air rises above this area and falls across the western half of the Indian Ocean basin, blowing westerly winds along the equator
  • Positive Phase of IOD:
    • During this phase the Westerly winds weaken along the equator, allowing warm water to move to Africa. Changes in the winds also allow a rise of cool water from the deep ocean in the east. This creates a temperature difference across the tropical Indian Ocean with cooler than normal water in the east and warmer than normal water in the west. This event has been found to be beneficial for the monsoon.
  • Negative Phase of IOD:
    • During this phase westerly winds intensify along the equator, allowing the concentration of warmer waters near Australia. This creates a temperature difference across the tropical Indian Ocean, with warmer than normal water in the east and cooler than normal water in the west. This event obstructs the progression of monsoon over India.

PRACTICE QUESTION:

With reference to 'Indian Ocean Dipole (IOD)' sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct?

1. IOD phenomenon is characterised by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.

2. An IOD phenomenon can influence an El Nino's impact on the monsoon.

Select the correct answer using the code given below:

(a) 1 only

(b) 2 only

(c) Both 1 and 2

(d) Neither 1 nor 2

Answer